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We explore the properties of the submillijansky radio population at 20 cm byapplying a newly developed optical color-based method to separate star forming(SF) from AGN galaxies at intermediate redshifts (z<1.3). Although opticalrest-frame colors are used, our separation method is shown to be efficient, andnot biased against dusty starburst galaxies. This classification method hasbeen calibrated and tested on a local radio selected optical sample. Givenaccurate multi-band photometry and redshifts, it carries the potential to begenerally applicable to any galaxy sample where SF and AGN galaxies are the twodominant populations.  In order to quantify the properties of the submillijansky radio population,we have analyzed ~2,400 radio sources, detected at 20 cm in the VLA-COSMOSsurvey. 90% of these have submillijansky flux densities. We classify theobjects into 1) star candidates, 2) quasi stellar objects, 3) AGN, 4) SF, and5) high redshift (z>1.3) galaxies. We find, for the composition of thesubmillijansky radio population, that SF galaxies are not the dominantpopulation at submillijansky flux levels, as previously often assumed, but thatthey make up an approximately constant fraction of 30-40% in the flux densityrange of ~50 microJy to 0.7 mJy. In summary, based on the entire VLA-COSMOSradio population at 20 cm, we find that the radio population at these fluxdensities is a mixture of roughly 30-40% of SF and 50-60% of AGN galaxies, witha minor contribution (~10%) of QSOs.

A new method to separate star forming from AGN galaxies at intermediate redshift: The submillijansky radio population in the VLA-COSMOS survey